The long-term objective of this research is to characterize the role of relative intensity of speech segments in the auditory processing of vowels and consonants within larger prosodic domains such as words. The role of intensity in speech processing has not been explored in a systematic way and its exact contribution is unknown. This application focuses on vowels and investigates the relative amplitude of vowel formants as a cue to making phonemic distinctions. Two specific aims are: 1) to examine the acoustic pattern of the relative amplitude of formants in the production of selected coarticulated and isolated vowels; and 2) to examine the auditory effect of manipulation of formant levels on listeners' decision about vowel quality and intelligibility of words. The proposed acoustic study is an extension of an early work on vowel intensity by House & Fairbanks (1953) to formant levels (F1, F2, F3, F4). Its goal is to establish whether and how vowel intensity is distributed over formant levels and whether this distribution varies with consonantal context. The proposed perception study investigates listeners' response to changes in the relative amplitude of formants for isolated vowels and for vowels in word context. Pilot data show that intensity of formants is a cue to vowel categorization. 24 subjects will be used to further test the patterns of formant levels in a larger set of American English vowels. The acoustic and perceptual relevance of these patterns aims to provide basic understanding about the way the vowel cues are integrated in a single percept when temporal and contextual factors reduce time for their processing in natural speech. Since the dynamic range in the intensity of sounds has been shown to have a dramatic effect on hearing aid performance, and since around one half of the Americans over the age of 65 may be candidates for amplification this research has implications for the quality of health care for these individuals.